1. Field of the Invention
This invention relates to a flash device arranged to be capable of performing flat emission in which emission of light is continued at a uniform quantity of light when a camera makes a slit exposure with a focal plane shutter, and more particularly to a control over the light emission intensity of the flat emission and a device for carrying out the control.
2. Description of Related Art
It has been practiced to carry out the flat emission in the following manner. A discharge tube is caused to emit light with the energy of electric charge of a main capacitor. When the emission intensity of the discharge tube comes to exceed a setting value, an intensity control means for controlling the intensity of light emission of the discharge tube sends an emission stop signal to an emission control means, composed of an IGBT circuit, etc., for controlling the light emission of the discharge tube. Upon receipt of the emission stop signal, the emission control means causes the discharge tube to stop emitting light. When the emission intensity drops below the setting value due to the pause of the light emission of the discharge tube, the intensity control means sends an emission start signal to the emission control means to cause the discharge tube to resume its light emission. These actions are repeated in this manner to continue the flat emission.
The setting value to be used for control over the intensity of light emission is set, for example, at an emission intensity setting value 303 as shown in FIG. 11(b), and the light emission is fixedly controlled by the intensity control means without varying the setting value until the end of the light emission.
In the case of the conventional method described above, the flat emission is controlled according to a fixed setting value during the light emission, as indicated by the setting value 303 in FIG. 11(b). Therefore, according to the conventional method, the waveform of light emission becomes aslant downward to the right as shown in FIG. 11(a). The slanting waveform is caused for the reason that the charging voltage of the main capacitor varies between a point of time 301 of the start of light emission and a point of time 302 of the end of light emission. As shown in FIGS. 12(a) and 12(b) which show the waveform of the light emission and the control signal obtained at the start of light emission and at the end of light emission, respectively, a difference in quantity of light emission between a point of time when the emission stop signal (a changing point from a high level to a low level of the control signal 902 or 904) is received and another point of time when the light emission intensity actually begins to drop is smaller at the end of light emission than at the start of light emission. FIG. 12(a) shows such a light quantity difference 905 obtained at the start of light emission while FIG. 12(b) shows such a light quantity difference 906 obtained at the end of light emission. As apparent from FIGS. 12(a) and 12(b), the difference 906 is smaller than the difference 905.
Therefore, as indicated by the waveform of light emission in FIG. 11(a), the intensity of light emission becomes weak at the time of point 302 of the end of light emission as compared with the point of time 301 of the start of light emission. The conventional method for the flat emission, therefore, has presented a problem that the film is exposed to a less quantity of light at the end of light emission than at the start of light emission.